Poly(dimethylsiloxane)(PDMS) is the first choice of material for a wide range of applications like microfluidic system, micro-electromechanical system, soft lithography and unconventional nanolithography, because PDMS has many advantageous properties such as nontoxicity, transparency, flexibility and chemical inertness. Furthermore, PDMS is widely used in many industrial areas such as aviation, electric, medical apparatus. Surface modification of PDMS is needed in order to play the best features of PDMS based devices and further promote the application of PDMS. A number of strategies have been developed for PDMS surface modification, which can be divided into two categories, namely physisorption and chemical coupling.
Physisorption of materials to PDMS surface, such as surfactants and polyelectrolytes are driven by hydrophobic force and electrostatic force, respectively. Although this method is simple, the surface film obtained is less stable and affected by the environmental changes such as temperature and pH change. And, the value of the density and thickness for the surface film are low, and limited to 4 gcm−2 and the range of 1 to 5 nm, so that it could be only applied to the very complicated environment such as the diluted serum, the pretreated the cell broken solution etc., or be used in the condition of relative stable environment and low shear force.
Chemical coupling can radically overcome the defect that the film absorbed by physisorption is unstable and is stable but is difficult to achieve because PDMS is chemically inert, which is ironically one of its merits. Common for this method, published in the literature: Dozel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel B.; Hilborn, J.; Delamarche, E. Adv. Mater. 2001, 13, 1164., the first step is to apply high-energy bombardment (i.e., plasma) to PDMS surface, which results in a silicate layer with functional groups on the surface, such as —OH, —COOH, and —NH2. Those functional groups not only render the surface hydrophilicity but also allow further modification via chemical coupling. Chemical coupling has two problems: (1) Plasma treatment is easy but not sustainable; recovery of hydrophobicity of treated PDMS is well documented. High-energy bombardment also has the tendency to damage PDMS. Furthermore, this strategy is only applicable to planar structure because of its limited penetration depth. (2) Concentration gradient in “grafting to” strategy prevents the preparation of thick and dense films.
Traditional physisorption and chemical coupling can not satisfy with the requirement of PDMS. Another method, designing and composing new materials, is advanced by the researchers. New materials have been developed to replace PDMS. For example, a photocurable perfluoropolyether (PEPEs) was synthesized to fabricate microfluidic devices that were organic solvent compatible. This material was published in the literature: J. P. Rolland, R. M. Van Dam, D. A. Schorzman, S. R. Quake, J. M. DeSimone, J. Am. Chem. Soc. 2004, 126, 2322. However, this method needs researchers develop a new material for every application. In a consensus, it is costly to develop a new elastomer for each individual need. Development of Micro-electromechanical system (MEMS) is a main direction of application of PDMS. Improving the biocompatibility of PDMS has been a hotspot. However, the material which meets the main property (mechanical property) and surface property (biocompatibility) to replace PDMS has not been reported up to now. Surface modification is an important factor to impact the application of materials and the performance. It can introduce new surface properties as necessary and retain the excellent performance of the main materials at the same time. And it is a high efficient and low consumed solution.